The present invention relates to high pressure hydraulic pumps, and particularly to pumps for supplying diesel fuel at high pressure in a fuel injection system for vehicles.
Rotary hydraulic pumps for use in diesel fuel injection systems for internal combustion engines, have been well known for a number of years. Recently, desired improvements in fuel efficiency and emissions control, have led the automotive industry toward development of so-called common rail fuel injection systems, whereby a high pressure pump is utilized to establish and maintain a high fuel pressure in an accumulator in fluid communication with individual injectors. Individual injection events are controlled at the injectors for achieving combustion in the individual combustion chambers of the internal combustion engine. This is in contrast to the more common distributor type fuel injection pumps, whereby fuel pulses are distributed from within the pump, to individual distribution paths leading to a respective plurality of injectors.
Common rail pumps are expected to operate at about 20,000 psi, whereas conventional distributor pumps operate at less than about 10,000 psi. This difference accentuates certain drawbacks in conventional pumps, such as an excessive amount of fuel that experiences pressurization in connection with the pumping action, and the excessive amount of heat carried by fuel which pressure pumping, but which is not actually injected into the combustion chambers.
Unfortunately, many of the disadvantages of distributor type pumps in this regard, have been carried over into attempts to modify the distributor type pumps, for use in common rail systems. The problem of excess pumping and associated heat generation, arise especially in the so-called pump-spill, spill-pump-spill, and fill-spill techniques, as exemplified in commonly owned U.S. Pat. No. 5,215,449 and co-pending U.S. application Ser. No. 08/459,032 filed Jun. 2, 1995. The reluctance in setting aside such spill-type pumps, is that the fuel delivery requirements on the pump can vary considerably depending on, for example, whether the pump is starting from a cold condition, whether the pump is running at a sustained, steady state condition, and whether acceleration is required to handle an increased load. With the spill-type pumps, a quantity of fuel is delivered to the pump in an amount greater than any necessary requirement, and spill control is utilized during pumping, to try to match the quantity discharged from the pump, with the instantaneous requirements.
Other techniques attempt to match fuel quantities delivered to the pumping chamber, with the instantaneous requirements, e.g., pre-metering based on computations of pump demand by an electronic control unit (ECU). This pre-metering of the fuel quantity to be charged into the pumping chamber is typically controlled by a solenoid valve responsive to a control signal from the ECU. A major disadvantage of solenoid-implemented pre-metering, is the relatively long duration required for the metering of a useful quantity of fuel through the solenoid valve, and the difficulty to adjust the metered quantity over a wide range according to the needs of the engine. In many instances, the intake phase of pumping chamber operation with pre-metering, would not leave sufficient time to implement the pumping phase using a cam pumping rate profile shallow enough to assure quiet operation. Even with dual-rate pumping profiles, there is not enough time available during the pumping phase of a cycle, to incorporate such duality.